A method of the above-mentioned type is disclosed in German published patent application DE-OS No. 17 03 681 and is based upon the realization that the slope of the stress-strain characteristic of a test rod falls off when the elastic limit is reached. The slope which is characteristic for this threaded fastener case can be determined in preliminary tests based on measurements of several like threaded fastener joints. Accordingly, a predetermined slope can be established for which the elastic limit is reached and the fastener driving tool switched off. This switch-off slope is utilized as a switch-off criteria for which the slope of the torque/ rotational-angle characteristic is determined during the fastener driving operation with respect to series fastening. This slope of the torque/rotational-angle characteristic is compared with the predetermined switch-off slope to deliver a control signal to switch off the fastener driving tool when the elastic limit is reached.
A precondition of such a method is that the course of the torque/rotational-angle characteristic for the individual fastener driving operations of a fastener case is for the most part the same. However, in practice there are often large deviations from the assumed characteristic curve to be determined without the fastener connection being defective. In these cases, the elastic limit is not reached or is greatly exceeded which is a disadvantage with respect to the integrity of the fastener connections or with respect to utilizing the threaded fasteners again.
In order to obtain an accurate switch-off of the fastener driving tool when the elastic limit is reached independently of the steepness of the characteristic, it is also known to not only determine the slope of the torque/rotational-angle characteristic in its linear portion as disclosed by the known method, but also to store and to generate a control signal as soon as the measured slope falls off to a predetermined percentage of the stored slope. In this connection, reference may be made to German Pat. No. 23 36 896. This method requires that a great accuracy of measurement be maintained when the slope is determined so that the desired switch-off value can be maintained. Furthermore, without special measures, errors cannot be detected which occur with large deviations from normal fastener driving conditions, for example, wherein the thread strips or a "chewing" of the fastener head occurs and which results in a substantially larger linear slope of the torque/rotational-angle characteristic.